A light-emitting diode (LED) has widely been put into practice as a light-emitting element due to p-n junction of a compound semiconductor, and has mainly been used in optical transmission, display and lighting applications. Since white LED has insufficient energy conversion efficiency as compared to an existing fluorescent lamp, there is a need to perform significant improvement in efficiency to general lighting applications. There remain many issues in realization of LED having high color rendering properties, low cost, and large luminous flux.
Currently marketed white LEDs are commonly equipped with a blue light-emitting diode element mounted on a lead frame, a yellow phosphor layer formed of YAG and Ce covered with this blue light-emitting diode element, and a molded lens formed of a transparent material suelemental substancech as an epoxy resin, which covers them. In the white LEDs, when blue light is emitted from the blue light-emitting diode element, blue light is partially converted into yellow light in the case of passing through the yellow phosphor. Since blue color and yellow color have complementary color relation to each other, blue light and yellow light are mixed to obtain white light. In the white LEDs, there is a need to perform an improvement in performances of the blue light-emitting diode element so as to improve efficiency and to improve color rendering properties.
There has been known, as the blue light-emitting diode element, a blue light-emitting diode element comprising, on an n-type SiC substrate, a buffer layer formed of AlGaN, an n-type GaN layer formed of n-GaN, a multiple-quantum-well active layer formed of GaInN/GaN, an electron blocking layer formed of p-AlGaN, and a p-type contact layer formed of p-GaN stacked successively from the SiC substrate side in this order. In this blue light-emitting diode element, a p-side electrode is formed on a front surface of the p-type contact layer and also an n-side electrode is formed on a back surface of the SiC substrate, and an electric current is allowed to flow by applying a voltage between the p-side electrode and the n-side electrode, whereby, blue light is emitted from the multiple-quantum-well active layer. Here, since the SiC substrate has conductivity, unlike the blue light-emitting diode element using a sapphire substrate, it is possible to dispose electrodes one above the other, and to attempt to make simplifications to the manufacturing process, in-plane uniformity of an electric current, effective utilization of a light-emitting area to a chip area, and the like.
There has also been proposed a light-emitting diode element which produces white light alone without utilizing a phosphor (see, for example, Patent Literature 1). In this light-emitting diode element, a fluorescent SiC substrate including a first SiC layer doped with B and N and a second SiC layer doped with Al and N is used in place of the n-type SiC substrate of the above-mentioned blue light-emitting diode element, thus emitting near-ultraviolet rays from the multiple-quantum-well active layer. Near ultraviolet rays are absorbed by the first SiC layer and the second SiC layer, and thus near-ultraviolet rays are converted into visible rays ranging in color from green to red in the first SiC layer and near-ultraviolet rays are converted into visible rays ranging in color from blue to red in the second SiC layer, respectively. As a result, white light having high color rendering properties near the sunlight is emitted from the fluorescent SiC substrate.